Many commercial buildings, e.g., hotels, office buildings and the like, have a hydronic heating system which includes a boiler (or, more typically, plural boilers)--the source of heat--and a steam/hot water circulating system delivering heated water into heat radiators in the building rooms. A boiler has a burner exhaust stack and an outer "shell" or housing containing water with heat distribution tubes immersed therein. Each boiler has a nominal maximum shell temperature at a nominal operating pressure and a characteristic stack temperature below which the boiler is able to absorb heat. Often, such boilers are fired using fuel oil or natural gas.
A common example of an older type of boiler heating system uses cast iron heat radiators. Newer systems use base board radiators. There are many such systems in service and a large percentage of them were installed when there was little concern about the cost of energy.
Such systems may have one or more heating "loops," each consisting of a hot water or steam output header from the boiler, one or more heat-dissipating radiators and a water return line to the boiler. In operation, very hot water or steam is pumped out of the header to the radiators where it "gives up" much of its heat into the space. The resulting cooler water flows along a return line back to a collection tank and thence to the boiler for reheating.
In systems having two or more heating or flow loops, a zone control valve is often installed in each loop. Such an arrangement permits separate control of the temperature in each zone in that, typically, a zone valve is opened to permit hot water flow only if that particular zone is "calling" for heat. A user-manipulated thermostat or temperature sensor with programmed set point initiates such a call when the actual temperature is at or below the set temperature.
As furnished by the manufacturer, boilers typically have two "firing rates," i.e., burner heat output rates such as exemplary rates of 300,000 and 800,000 BTU rates. Of course, the actual firing rates depend upon the size and configuration of boiler and other factors. Firing rate is controlled by controlling the flow of fuel to the burner using a butterfly or other type of gas valve.
Designers of boiler-driven hydronic heating systems have only superficially considered energy management (an environmental concern) and occupant comfort when configuring system control techniques--and such techniques are usually relatively crude. Simply stated, most such systems may be characterized as being of the on-off "brute strength" type. There is little attempt at energy conservation.
And that is not all. Existing control systems frequently fire a boiler at a firing rate which is excessive in view of the then-existing boiler water temperature. At water temperatures below about 160.degree. F., firing rate must be carefully controlled to avoid cracking boiler tubing. Even a so-called "low" firing rate can crack such tubing under certain conditions. In fact, "overfiring" a cooled boiler is the primary cause of boiler failure.
The device described in U.S. Pat. No. 4,516,720 (Chaplin) is based on the proposition that greatest efficiency results when a heating system is operated at or near 100% of the time. The Chaplin device adjusts the initial output temperature of the heat-carrying fluid, e.g. water, as a function of outdoor ambient temperature and of the percentage of time that a space thermostat is at or below a set point or "call for heat" temperature.
The temperature of the heat-carrying fluid is progressively adjusted upward until the space thermostat calls for heat a high percentage of the time, 90-95% as a practical matter. This condition represents an approximate balance between heat output from the system and heat dissipation into the space.
U.S. Pat. No. 4,941,609 (Bartels et al.) describes a method and apparatus for controlling firing rate. A parameter used for such control is a "thermal mass" figure which is a numerical indication of the heat absorbing capability of the boiler. Another parameter is an operator-input figure denoting the "tightness" or "looseness" with which temperature control is to be achieved. Apparently, this is merely another way of denoting the bandwidth of the temperature excursions which will be permitted to occur in the space. As assumed examples (which do not expressly appear in the patent), a "tight" mode of control may permit a 3.degree. F. bandwidth while a "loose" mode of control made permit an 8.degree. F. bandwidth.
U.S. Pat. No. 3,421,691 (Forbes) describes a system having a heat sink separate from (and in addition to) the system boiler. The temperatures of the heat sink and of the boiler water are used as parameters to control boiler firing rate to more slowly increase the temperature of the water circulating in the heating loops.
Clearly, there is a need for a new method for boiler control which can effect significant system operating savings while yet retaining an acceptable level of heating comfort and avoiding overly aggressive firing of a cool boiler.